Carbon Metabolism of C14-Labeled Amino Acids in Wheat Leaves. II. Serine & its Role in Glycine Metabolism

Abstract

Wang and Waygood (24) investigated the conversion of glycine to sugars in wheat leaves and proposed a glyoxylate-serine pathway in which serine occupies a key position as an essential intermediate. Rabson et al. (19) also have discussed the glycolate pathway for the production of precursors of hexoses. They have reviewed much of the literature on the metabolism by plants of glycolic acid, glyoxylic acid, glyceric acid, glycine, serine, and related intermediates. Glycolic acid is an early product of photosynthesis (1), and a substantial percentage of the total carbon of the photosynthetic cycle enters into glycolate (1,25). This was shown dramatically when Zelitch (27) blocked glycolate oxidation with an a-hydroxysulfonate and demonstrated that approximately half of the C14 fixed appeared in glycolic acid. Kearney and Tolbert (11) showed that C14-labeled glvcolate and phosphoglycolate appeared rapidly outside isolated chloroplasts photosynthesizing C1402, and they suggested that these compounds likely are of special importance in the transport of photosynthate between chloroplasts and cytoplasm. Glycine arises from glyoxylate and in turn serves as a precursor for serine. Rabson et al. (19) also have implicated glycerate as an intermediate between serine and hexoses, and have suggested that this glycerate formed by the glycolate pathway represents a precursor for hexoses independent of the phosphoglyceric acid formed from the carboxylation of ribulose diphosphate. In the present work, an attempt has been made to substantiate the role of serine in the metabolism of glycine as well as to investigate the glyoxylateserine pathway in general. Metabolic changes with time in a number of labeled substrates have been followed in an attempt to clarify the interrelationships between these simple organic acids and amino acids and their roles as precursors for sugar synthesis. Materials & Methods